Wideband chrominance signal filter

ABSTRACT

A color television receiver includes a comb filter responsive to detected video signals for providing a wideband combed chrominance signal, which is coupled to chrominance processing circuits via a composite filter including plural mutually interactive resonant sections. An input resonant section of the filter serves to remove very high frequency interference signals including comb filter switching components from the chrominance signal path, and also coacts with the other filter sections to provide an amplitude characteristic having a sense to compensate for an oppositely directed amplitude characteristic associated with the detected video signals, over the passband of the wideband chrominance signals.

This invention concerns an electrical signal filter in a colortelevision receiver system for imparting a prescribed amplitudecharacteristic to the chrominance component of the television signalprior to chrominance signal processing. In particular, the inventionconcerns such a filter in a system including provision for processing awideband chrominance information component.

In a color television signal according to NTSC broadcast standards suchas employed in the United States, the chrominance information componentof a color television signal encompasses signal frequencies fromapproxmately 2.08 MHz to 4.08 MHz, between the 3 db points, in themodulated chrominance frequency spectrum. The frequency of thechrominance subcarrier reference signal, approximately 3.58 MHz, issituated in the upper frequency portion of the modulated chrominancefrequency spectrum.

In "narrowband" chrominance signal processing, which is employed in manycolor television receiver systems, a narrow range of modulatedchrominance signal frequencies are processed to extract the colorinformation. Typically, such frequencies occupy a 1 MHz bandwidthcentered about the 3.58 MHz chrominance subcarrier frequency (i.e., 3.58MHz ±0.5 MHz). Narrowband chrominance processing has been found to be anacceptable, less complicated alternative under certain circumstances,compared to wideband chrominance processing.

"Wideband" processing of the chrominance component entails processingthe full 2.08 MHz to 4.08 MHz modulated chrominance signal bandwidth.Wideband chrominance processing is particularly advantageous in highdefinition color television signal systems, such as color receiversemploying comb filtering techniques for extracting the luminance andchrominance components from the composite color television signal priorto luminance and chrominance processing. With wide bandwidth chrominanceprocessing, greater use can be made of the available color informationcontent of the television signal, resulting in improved color picturedefinition and enhanced subjective color sharpness.

A choice of wideband or narrowband chrominance processing is oftenimplemented by means of a wideband or narrowband bandpass filterinserted between the output of the video detector stage of the receiver,which follows the intermediate frequency (IF) stage of the receiver, andthe chrominance signal processing circuits of the receiver. The responseof the IF stage is sometimes such that IF output signals, as supplied tothe video detector, exhibit a decreasing amplitude slope with respect tomodulated chrominance signal frequencies. This IF amplitude responsecharacteristic must be compensated for before subjecting the modulatedchrominance component to processing by the chrominance processingcircuits of the receiver. The chrominance bandpass filter which precedesthe chrominance processing circuits is a convenient place to providesuch compensation.

It is herein recognized that such compensation should produce arelatively flat amplitude characteristic over the frequency range of thewideband chrominance component, as well as a substantially symmetricalamplitude characteristic over a narrower range of chrominance signalfrequencies centered about the chrominance subcarrier frequency in theupper portion of the wideband chrominance frequency range. It isfurthermore recognized as desirable to use an uncomplicated, economicalchrominance bandpass filter to achieve these results. These objectivesare satisfied by a chrominance bandpass filter disclosed herein inaccordance with the principles of the present invention.

The disclosed bandpass filter receives input chrominance signalsincluding a subcarrier component at a frequency unsymmetrically disposedin the range of frequencies occupied by the chrominance signal. Theinput chrominance signal exhibits a decreasing amplitude slope withrespect to frequencies in the chrominance signal frequency range. Thebandpass filter exhibits a passband wherein the chrominance subcarrierfrequency is unsymmetrically disposed therein. The bandpass filterimparts to the chrominance signal an amplitude characteristic with anoppositely directed, increasing amplitude slope to compensate for thedecreasing amplitude slope associated with the input chrominancesignals. Output chrominance signals from the bandpass filter exhibitsubstantially equal amplitudes at the end frequency extremes of thewideband chrominance signal frequency range, a relatively flat amplitudecharacteristic over the wideband chrominance frequency range, and asubstantially symmetrical amplitude characteristic over a narrower rangeof chrominance signal frequencies centered about the chrominancesubcarrier frequency in the upper portion of the wideband chrominancefrequency range.

In accordance with a feature of the invention, the chrominance signalsreceived by the bandpass filter are derived from a composite videosignal by means of a comb filter which responds to switching signals. Aninput resonant section of the bandpass filter includes a capacitance forsuppressing high frequency signals such as may be associated with theswitched operation of the comb filter. The resonant input section alsocoacts with other resonant sections of the bandpass filter to produce apeaked filter amplitude response.

In the drawing:

FIG. 1 shows a portion of a color television receiver including abandpass filter network in accordance with the principles of the presentinvention; and

FIGS. 2-4 depict amplitude-versus-frequency characteristics helpful inunderstanding the operation of the filter network shown in FIG. 1.

In FIG. 1, broadcast color television signals including imagerepresentative luminance and chrominance components are received by anantenna 10 and applied to a television signal processing network 12 ofthe receiver. Network 12 includes radio frequency and intermediatefrequency (IF) signal processing stages, and a video detector stageresponsive to the IF signals. Detected video signals from network 12,including luminance an chrominance components, are applied to an inputof a comb filter network 20. In this example comb filter 20 comprisescharge coupled devices and can be of the type shown in U.S. Pat. No.4,096,516 for example. In comb filter 20 signal charge packets aretransferred (i.e., switched) from stage to stage in response to timingsignals from a source 25. The timing signals have a frequency of 10.7MHz, which corresponds to the third harmonic frequency of the 3.58 MHzchrominance subcarrier frequency. Comb filter network 20 is containedwithin a grounded conductive enclosure 21 which serves as a shield tosuppress radiation of radio frequency interference (RFI) signals such ascan be generated by the rapid amplitude transitions of the comb filterswitching signals.

A "combed" luminance signal from a first signal output of comb filter 20is coupled via a low pass filter 28 to an input of a signal combiningnetwork 35. Filter 28 is arranged to pass luminance signal frequenciesfrom DC to approximately 4.0 MHz, and serves to remove noise andswitching frequency components of the timing signals associated with theswitching operation of comb filter 20.

A "combed" chrominance signal from a second signal output of comb filter20 is applied to a low pass vertical detail filter 30. Filter 30exhibits an upper cutoff frequency of approximately 1.5 MHz, andselectively passes those signal frequencies present in the second signaloutput of comb filter 20 which lie below this cut-off frequency. Signalfrequencies in this region represent vertical detail luminanceinformation which is absent from the combed luminance signal nd whichmust be restored to the combed luminance signal to avoid loss ofvertical image resolution in the luminance content of a displayed image.Such vertical detail resolution is accomplished by combining, incombiner 35, an appropriate amount of the vertical detail signal fromfilter 30 with the filtered combed luminance signal from filter 28. Theoutput signal from combiner 35 corresponds to a reconstituted luminancecomponent of the color television signal, which is afterwards applied toa luminance signal processor 40.

Combed chrominance signals from the second output of comb filter 20 arealso coupled, via a resistor 24 and a conductor which passes through anaperture A in shielded enclosure 21, to a wideband chrominance bandpassfilter 45 according to the present invention as will be discussedsubsequently. Output signals from bandpass filter 45 are coupled via apre-set potentiometer 47 to a wideband chrominance signal processor 48for providing R-Y, G-Y and B-Y color difference signals. Chrominanceprocessor 48 includes gain and phase control networks, a controlledlocal oscillator for regenerating a 3.58 MHz color subcarrier referencesignal, synchronous "I" and "Q" chrominance signal demodulators, and Iand Q demodulator filter networks e.g., of the type discussed incopending U.S. pat. application Ser. No. 488,813 of S. V. Naimpally,titled "Demodulated Chrominance Signal Filter Using Impedance MismatchedSections", now U.S. Pat. No. 4,536,788. The luminance output signal (Y)from processor 40 and the color difference signals from processor 48 arecombined in a matrix amplifier 50 for providing R, G, B color imagerepresentative signals which are suitably coupled to image intensitycontrol electrodes of a color kinescope 55.

The combed chrominance signal as applied to chrominance bandpass filter45 includes mutually quadrature phased "I" and "Q" signal modulationcomponents. In a color television signal processing system according toNTSC broadcast standards such as employed in the United States, the Qchrominance signal modulation component occupies an approximately 0.5MHz bandwidth on both upper and lower sidebands symmetrical with respectto the 3.58 MHz frequency of the chrominance subcarrier signal. Thus inthe modulated chrominance frequency spectrum the Q chrominanceinformation includes signal frequencies from 3.08 MHz to 4.08 MHz. The Ichrominance signal modulation component occupies an approximately 1.5MHz bandwidth on a lower sideband relative to the chrominance subcarrierfrequency, and occupies a 0.5 MHz bandwidth on an upper sidebandrelative to the chrominance subcarrier signal frequency. Thus in themodulated chrominance frequency spectrum the I chrominance informationincludes signal frequencies from 2.08 MHz to 4.08 MHz. This range ofsignal frequencies also corresponds to the range of signal frequenciesencompassed by the wideband chrominance signal, and will hereinafter bereferred to as the wideband chrominance signal passband.

In many color television receivers the detected video signal fromnetwork 12 exhibits significant attenuaton of the chrominance signalamplitude over a significant portion of the chrominance passband,typically due to the characteristics of the IF stage. This effect isillustrated by FIG. 2 which shows the amplitude-versus frequencycharacteristic for signals applied to filter 45, and is substantiallycompensated for by means of composite bandpass filter 45 before thechrominance signal is subjected to processing in processor 48.

Filter 45 comprises an input resonant section 45a, an intermediateresonant section 45b, and an output resonant section 45c all of whichare mutually interactive. Chrominance signals are applied to inputsection 45a via a low impedance output of network 20 (e.g., via anemitter follower transistor stage associated with network 20), andresistor 24. Output chrominance signals from filter 45 are applied to ahigh impedance input of chrominance processor 48.

Referring to FIG. 3 for the moment, there is shown theamplitude-versus-frequency response characteristic of output signalsfrom bandpass filter 45 as incorporated in the receiver. Outputchrominance signals from filter 45 vary in amplitude by slightly lessthan 3 db over the chrominance passband from 2.08 MHz to 4.08 MHz, whichis acceptable with respect to the signal processing requirements ofchrominance processor 48. Chrominance signals at the 2.08 MHz and 4.08MHz end frequencies of the wideband chrominance passband aresubstantially equal in amplitude. Out-of-band signal frequencies above4.08 MHz are highly attenuated.

As seen from FIG. 3, the 3.58 MHz chrominance subcarrier frequency issituated in the upper frequency portion of the chrominance passband, andis thus non-symmetrically disposed in the chrominance passband. Theamplitude response produced by filter 45 at 3.58 MHz ±0.5 MHz isapproximately symmetrical, with the amplitude response at 3.08 MHz beingapproximately -1 db down relative to 3.58 MHz, and the amplituderesponse at 4.08 MHz being approximately -2 db down relative to 3.58MHz. Amplitude symmetry in this frequency range is desirable withrespect to the demodulation of the symmetrically double sidebanded Qcomponent of the chrominance signal, which in the modulated chrominancefrequency spectrum occupies a frequency range from 3.08 MHz to 4.08 MHz.

FIG. 4 illustrates the amplitude-versus-frequency response of bandpassfilter 45 alone. As seen from FIG. 4, filter 45 exhibits anapproximately 8 db rising amplitude characteristic over the chrominancepassband from 2.08 MHz to 4.08 MHz.

Returning to FIG. 1, input section 45a of filter 45 comprises a parallelresonant LC circuit including a capacitor 60 and an inductor 62 shuntingthe input chrominance signal path. Capacitor 60 and inductor 62 exhibita resonant frequency of 4.53 MHz, above the chrominance passband. Aswill be discussed subsequently, the input resonant circuit includingcapacitor 60 and inductor 62 is included to assure that the overallfilter response exhibits a rising amplitude characteristic over most ofthe chrominance passband to compensate for the declining amplitudecharacteristic manifested by signals applied to filter 45 from precedingstages. This is accomplished by the coaction of circuit 60,62 with thecomplex impedance manifested by the remainder of bandpass filter 45. Inaddition, capacitor 60 of the input section forms a low pass filter withseries input resistor 24 to suppress very high frequency radio frequencyinterference (RFI) signals generated by the switching action of combfilter 20, as will also be discussed.

Intermediate section 45b comprises a series resonant LC circuitincluding a capacitor 70 and inductors 71,72 in series with the signalpath. Capacitor 70 and inductors 71,72 exhibit a resonant frequency ofapproximately 1.8 MHz, below the chrominance passband. A capacitor 75coacts with inductor 71 to form a parallel resonant trap at 7.2 MHz (thesecond harmonics of the chrominance subcarrier frequency). Capacitor 76and inductor 72 coact to form a parallel resonant trap at 10.7 MHz (thethird harmonic of the chrominance subcarrier frequency). Capacitors 75and 76 have substantially no effect upon the bandpass filter response atthe relatively lower frequencies within the chrominance passband.

Output section 45c comprises a parallel resonant LC circuit including acapacitor 80 and an inductor 82 shunting the output chrominance signalpath. Capacitor 80 and inductor 82 exhibit a resonant frequency ofapproximately 4.3 MHz, above the chrominance passband.

Thus sections 45a, 45b and 45c are each resonant at a frequency outsideof the chrominance passband. Since greater delay is experienced atresonant frequencies, setting the resonant frequencies of the respectivefilter sections outside of the chrominance passband prevents suchotherwise experienced greater delay from impairing the overall delayresponse of the composite bandpass filter.

Mutual coupling between the parameters of the filter sections occursbecause the filter sections are not isolated from one another. Thus, forexample, sections 45b and 45c coact somewhat such that section 45bexhibits an effective resonant frequency of approximately 1.6 MHz, andsection 45c exhibits an effective resonant frequency of approximately4.7 MHz, both frequencies shifted slightly from the individal resonantfrequencies of these sections as mentioned previously. However, theseshifted resonant frequencies remain outside of the chrominance passband.An amplitude peak produced at approximately 4.7 MHz, as shown in FIG. 4,results from the coaction of filter sections 45b and 45c.

Capacitor 60 and inductor 62 of input section 45a coact with the compleximpedance presented by filter sections 45b and 45c to produce a peakedamplitude response at approximately 1.35 MHz, as shown in FIGS. 3 and 4.That is, the previously mentioned low end resonant frequency of 1.6 MHzis shifted downward to 1.35 MHz. This peaked amplitude responsedesirably yields a rising amplitude characteristic over the entirewideband chrominance passband encompassing 2.08 MHz to 4.08 MHz, asshown in FIG. 4. Such rising amplitude characteristic assists to providesuitable compensation for the decreasing amplitude response manifestedby the input signals applied to filter 45 (FIG. 2) due to the IF signalprocessing characteristics of signal processor 12 over the widebandchrominance passband. The overall amplitude response of signals fromfilter 45 in the system of FIG. 1 consequently exhibits an acceptablyflat amplitude characteristic over the wideband chrominance passband, asindicated by FIG. 3. As noted previously, amplitude symmetry in thefrequency range of 3.58 MHz ±0.5 MHz also advantageously results.

Capacitor 60 of the input section also forms a low pass filter withinput resistor 24 to suppress very high frequency RFI signals generatedby the switching action of comb filter 20, as follows.

Resistor 24 is enclosed by metallic enclosure 21 which shields combfilter 20. Capacitor 60 is located external to, but in close physicalproximity with, a planar surface 23 of enclosure 21 from which signalsemerge via resistor 24 and an output aperture A. Capacitor 60 comprisesa ceramic disc capacitor with a ceramic wafer dielectric located betweena positive or "hot" conductive planar plate connected to a node "B", anda negative (less positive) conductive planar plate coupled to the samesource of ground reference potential as enclosure 21. The "hot" plate ofcapacitor 60 is situated substantially in parallel with and facingsurface 23 of enclosure 21.

Without capacitor 60, the portion of the conductor which couplesresistor 24 to node B outside of enclosure 21 would act as an antennafor RFI energy, radiating this energy to nearby circuits. However,capacitor 60 prevents this radiation by conducting a portion of the RFIenergy through the capacitor dielectric to the grounded plate ofcapacitor 60. The balance of the RFI energy at node B and at theconnections to this node is radiated from the surface of the positive or"hot" plate of capacitor 60 toward surface 23. The RFI energy radiatedfrom the positive plate is narrowly confined to the area betweencapacitor 60 and surface 23, and is returned to ground via enclosure 21.In essence, the positive plate of capacitor 60 and the facing portion ofsurface 23 comprise an air-dielectric capacitance for bypassing the RFIenergy from the signal path. Thus the arrangement of capacitor 60 actsas a low impedance bypass for most of the RFI energy at node B,conducting it harmlessly to ground.

This mechanism eliminates RFI radiation of very high frequencycomponents such as are associated with the rapid amplitude transitionsof the 10.7 MHz comb filter switching signals including higher harmonicsof such switching signals. Such very high frequency components extendthrough the IF, VHF and UHF bands of the radio frequency spectrum, andcan intermodulate with received television signals to cause severepicture beat patterns on several television channels. Additional detailsof this RFI suppression technique are found in U.S. Pat. No. 4,267,528of G. E. Thornberry, titled "Radio Frequency Interference SuppressionApparatus.

Thus capacitor 60 advantageously serves both as a low pass RFI filterelement for suppressing switching transients from comb filter 20, aswell as comprising a part of bandpass filter 45 for providing thedesired wideband chrominance amplitude-versus-frequency characteristicas described.

What is claimed is:
 1. In a system for processing a video signalincluding a wideband chrominance signal component comprising achrominance subcarrier component exhibiting a frequency unsymmetricallydisposed in the range of frequencies occupied by said chrominancesignals, apparatus comprising:a source of video signals exhibiting anamplitude chacteristic with a decreasing slope over a frequency rangeincluding frequencies associated with said wideband chrominancecomponent; comb filter means responsive to said video signals and toswitching signals for deriving said wideband chrominance component fromsaid video signal, said derived chrominance component exhibiting saidamplitude characteristic of decreasing slope; and filter meansresponsive to said derived wideband chrominance signals and exhibiting apassband wherein said chrominance subcarrier frequency isunsymmetrically disposed in said passband, for providing outputchrominance signals in accordance with the amplitude-versus-frequencyresponse of said filter means; wherein said filter means imparts to saidchrominance signals an amplitude characteristic with an increasing slopefor substantially compensating for said amplitude characteristic withsaid decreasing slope; said filter means comprises an input resonantsection including a capacitance for suppressing high frequency signalsabove the range of chrominance signal frequencies; a resistance couplessaid derived chrominance signals to said input section of said filtermeans, said resistance forming a low pass filter with said capacitance;said filter means comprises plural mutually interactive resonantsections including said input section; said input section comprises aninductance arranged as a parallel resonant circuit with saidcapacitance; and said input section coacts with other of said pluralresonant sections to produce a peaked filter amplitude response at afrequency below said range of chrominance signal frequencies.
 2. In asystem for processing a video signal including a wideband chrominancesignal component comprising a chrominance subcarrier componentexhibiting a frequency unsymmetrically disposed in the range offrequencies occupied by said chrominance signals, apparatus comprising:asource of video signals exhibiting an amplitude characteristic with adecreasing slope over a frequency range including frequencies associatedwith said wideband chrominance component; means for deriving saidwideband chrominance component from said video signal, said derivedchrominance component exhibiting said amplitude characteristic ofdecreasing slope; and filter means responsive to said derived widebandchrominance signals and exhibiting a passband wherein said chrominancesubcarrier frequency is unsymmetrically disposed in said passband, forproviding output chrominance signals in accordance with theamplitude-versus-frequency response of said filter means; wherein saidfilter means imparts to said chrominance signals an amplitudecharcteristic with an increasing slope for substantially compensatingfor said amplitude characteristic with said decreasing slope; and saidfilter means comprises plural mutually interactive resonant sectionsincluding an input resonant section; an output resonant section; and anintermediate resonant section coupling said input section to said outputsection.
 3. Apparatus according to claim 2, whereinsaid input sectioncomprises a capacitive input network.
 4. Apparatus according to claim 2,whereinsaid input, output and intermediate resonant sections areindividually tuned to frequencies outside of said range of chrominancesignal frequencies.
 5. Apparatus according to claim 4, whereinsaid inputand output resonant sections are individually tuned to frequencies abovesaid range of chrominance signal frequencies; and said intermediateresonant section is individually tuned to a frequency below said rangeof chrominance signal frequencies.
 6. Apparatus according to claim 5,whereinsaid input resonant section coacts with said intermediate andoutput resonant sections to produce a peaked filter amplitude responseat a frequency below said range of chrominance signal frequencies. 7.Apparatus according to claim 2, whereinsaid input section comprises aparallel resonant inductance-capacitance network coupled in shunt withthe chrominance signal path; said intermediate section comprises aseries resonant inductance-capacitance network coupled in series withthe chrominance signal path; and said output section comprises aparallel resonant inductance-capacitance network coupled in shunt withthe chrominance signal path.
 8. Apparatus according to claim 7,whereinsaid intermediate section comprises plural inductances eachrespectively associated with a capacitance for providing plural trapswith respect to harmonic frequencies of said frequency of saidchrominance subcarrier.